The treatment of CNS diseases and ocular disorders are worldwide important fields of research. Several heterocyclic compounds have shown to interact with neurotransmitters which are released by neurons of the central nervous system (CNS). Some compounds have been tested for the treatment of CNS-disorders such as Alzheimers disease. Scientific studies have also shown that glaucoma is a leading cause of blindness. One pathologic sign of glaucoma is the progressive degeneration of retinal ganglion cells and their axons which form the optic nerve. The classification of glaucoma also includes the following types:                Primary angle-closure glaucoma, secondary open-angle glaucoma, steroid-induced glaucoma, traumatic glaucoma, pigmentary dispersion syndrome, pseudo-exfoliation syndrome, secondary angle-closure glaucoma, neovascular glaucoma, uveitis and glaucoma and other non further specified eye pathologies.        
In addition, age-related macular degeneration is a typical condition which has features of glaucoma and leads to a progressive loss of vision, leading finally to blindness. The treatment of ocular diseases includes the treatment of elevation in the intraocular pressure (IOP) over a normal range. Many individuals with clearly have elevated IOP do not develop glaucoma, and many patients with glaucoma do not have an increased IOP.
Currently available medications/drug compounds for the treatment of ocular diseases, in particular glaucoma, belong to several pharmacological classes, including β-adrenergic blockers, cholinergic agonists, carbonic anhydrase inhibitors and alpha agonists. All of them operate under a mechanism whereby the IOP is lowered. These existing medications are typically administered locally, e.g. as eye drops. Hyperosmotics may also be administered intravenously for emergency treatment. In addition, laser therapy and surgical approaches are applied in special cases of ocular diseases.
There is however an unmet medical need for better pharmaceutical drug compounds and alternative treatment strategies. Particularly for patients with progressive glaucomatous damage under normalized IOP, a drug therapy focusing on the rescue of degenerating retinal ganglion cells is needed. A particular need is for stable drug compounds which easily can be applied to humans and other mammals.
There are different scientific theories regarding the causes for the degeneration of the retinal ganglion cells including mechanical, vascular and excitotoxic mechanisms. The β-amyloid peptide has been found to co-localize with dying retinal ganglion cells [see Yoneda S, “Vitreous fluid levels of beta-amyloid (1-42) and tau in patients with retinal diseases”, Jpn. J. Ophthalmol. 2005, 49(2) p.106-108]. Furthermore, animal studies demonstrated that the soluble Aβ1-42 peptide oligomers are potent toxins for retinal ganglion cells [see Guo L, “Targeting amyloid-β in glaucoma treatment”, PNAS 2007, 104(33), p.13444-13449]. This study of L. Guo showed that inhibition of aggregation of Abeta reduces glaucomatous degeneration of retinal ganglion cells. The inhibitors used in the animal experiments were known compounds, such as the diazo-biphenyl-derivative Congo red and Abeta antibodies. These agents however are pharmacological research tools only. Abeta antibodies are known to block Abeta aggregation specifically, however the usefulness of anti-Abeta antibodies for the treatment of glaucoma in humans is limited by known side effects.
Some β-Secretase inhibitors can have beneficial effects on Abeta-related neurotoxicity, however the observed effects in rat retinal ganglion cells were not significant. In the literature, various types of substituted indole compounds have been disclosed which have interesting pharmaceutical properties. Some known peptidic indole derivatives can be used for pharmaceutical purposes, such as the treatment of diabetes, Alzheimers disease and others [see e.g. WO2005/000193 and WO2009/024346]. Also, neuro-protective pharmaceutical compositions have been described [see WO2003/063760 and WO2003/077869]. Several compounds that inhibit Abeta polymerization and which are effective in animal models are described in the scientific literature, e. g. cyclohexanehexol compounds [see J. Mc Laurin, Nature Medicine 12(7), 2006, p. 801-808].
Solutions of the phenolic yellow curry pigment curcumin were found to inhibit Abeta aggregation in vitro [see F. Yang, Journal of Biological Chemistry 208(7), 2005, p. 5892-5901].
However, the substances described in the prior art, are often not sufficiently active in inhibiting Abeta aggregation and/or polymerization or they have unwanted side-effects.
In the publication of Y. K. Shue, “Double bond isosteres of the peptide bond: Synthesis and biological activity of cholecystokinin (CCK) C-terminal hexapeptide analogs” (Bioorganics & Medicinal Chemistry 1, No. 3, 1993, 161-179) several indol compounds are described, which can be used for this synthesis of tetra-peptides.
As one compound, the structure (D1) is shown.

In the publication of B. E. Kornberg “Synthesis of TRP-VAL non-cleavable dipeptide transition state isosteres” (Bioorganics & Medicinal Chemistry 3, No. 6, 1993, 1257-1262), a multiple step preparation is described. In this article, the following reaction is shown to lead to the structure (D2).

In the publication of Maria Teresa Garcia-Lopez “Synthesis and Inhibitory Activities against Aminopeptidase B and Enkephalin-Degrading Enzymes of Ketomethylene Dipeptide Analogues of Arphamenines” (Archiv der Pharmazie, 325, No. 1, 1992, 3-8) various dipeptide compounds are described which have inhibitory activities against Aminopeptidase B.
In the publication of Maria Teresa Garcia-Lopez “Synthesis of ketomethylene dipeptides containing basic amino acid analogues at C-terminus” (Tetrahedron, 44, No. 16, 1988, 1531-1538) several indol-derivatives are described such as the following compound (D4).

In the international patent application WO 1988/03927 various types of Renin-inhibitory peptides are disclosed, which as one amino acid can comprise L-tryptophan. In the document WO 2005/060683, several types of small peptides are described which can be useful for the treatment of Alzheimers disease. Several of the peptides disclosed can comprise a tryptophan structure.